The Release 10 Working Item in 3GPP (3rd Generation Partnership Project) standardization on LTE (Long Term Evolution) carrier aggregation for LTE mainly focused on LTE FDD (Frequency Division Duplex) (cf. document [1]). In Release 11, further mechanisms will be addressed to support inter-band carrier aggregation for LTE TDD DL and UL including different uplink-downlink configurations on different bands (cf. document [2]).
FIG. 1 illustrates an example of TDD inter-band CA that supports different TDD DL/UL configurations.
In current discussion in 3GPP, e.g. RAN1#66bis meeting, half duplex mode TDD UE (User Equipment) is possible to be regarded as the baseline solution for inter-band CA with different TDD DL/UL configurations. That is to say, half duplex mode TDD UE are not allowed to do simultaneous transmission and reception in an overlapped subframe, although it may have two sets of RF (Radio Frequencies). Thus, the transmission direction of the overlapped subframe needs to be decided when TDD UE copes with such subframes. One possible way forward is to use the similar method as FDD HD (Half Duplex) UEs, i.e. dynamically determining the transmission direction of the overlapped subframes according to UL grant or UL feedback, which is proposed in document [3].
This method is the possible way forward since it is already used in previous FDD HD mode. And now for TDD inter-band CA, one problem is how to cope with RACH transmission in the overlapped subframe.
According to the present invention there is proposed one RACH enhancement method for inter-band TDD CA with different DL/UL configuration.
According to the overlapped subframe transmission direction scheme as mentioned above, UE will regard overlapped subframe as UL subframe, if there is RA (Random Access) preamble transmission or RA Msg3 transmission. But meanwhile, eNB is not aware if UE is going to transmit RA preamble and RA Msg3 for contention-based RA in which UE will randomly select one RA preamble and decide the timeslot of preamble transmission by itself. Thus, it is still possible that the eNB schedules DL transmission in the overlapped subframe. Then, this DL assignment will be missed by UE. This is illustrated in FIG. 2.
In FIG. 2, for overlapped subframe in black block, if there is a RA-preamble to be transmitted, UE will regard this overlapped subframe as UL subframe, i.e. UE will transmit RA preamble in the UL subframe and ignore DL subframe of this overlapped subframe. Since eNB is not aware of the PRACH transmission from UE, when eNB transmits a DL grant in the same subframe, it may consider that the DL grant is missed at the UE. As a result, eNB may increase the PDCCH (Physical Downlink Control Channel) aggregation level for better performance or even trigger some updates to PDCCH OLLA (Open Loop Link Adaptation). Then OLLA performance is impacted and more resource could be wasted.
Such an issue at least exists in the following scenarios.
The first scenario relates to a D-SR failure. In such a case, the UE initiates RACH on UL subframe of overlapped subframe. Then, UL subframe is prioritized, and DL subframe is ignored.
The second scenario relates to a case where the eNB triggered a contention-based RA. In such a case, the eNB triggered a contention-based RACH, and UE initiates RACH on UL subframe of overlapped subframe. This case could be happened when e.g. eNB triggered RACH on Scell to get TA on Scell in multiple-TA scenario in CA.